Subwavelength optical microstructure light collimating films

ABSTRACT

A light collimating film has a sheeting having a first side and a second side, wherein the first side includes a series of linear optical elements having a primary axis running the length of the optical elements, and the second side includes a plurality of subwavelength optical microstructures being oriented at about 90 degrees relative to the primary axis of the linear optical elements. Another embodiment includes a back lighting display device having a lighting device, a display panel, and a sheeting having a first side and a second side, wherein the first side includes a series of linear prisms having peaks, and the second side includes a plurality of subwavelength optical microstructures being oriented at about 90 degrees relative to the peaks of the linear prisms. Yet another embodiment includes a light collimating structure having a first collimating film having a first surface with a plurality of subwavelength optical microstructures thereon and a second surface with first linear prisms having peaks, the subwavelength optical microstructures being oriented at about 90 degrees relative to the peaks of the first linear prisms, and a second collimating film having a first surface with a plurality of subwavelength optical microstructures thereon and a second surface with second linear prisms having peaks, the subwavelength optical microstructures being oriented at about 90 degrees relative to the peaks of the second linear prisms.

RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 09/438,912, filed Nov. 12, 1999, now U.S. Pat. No.6,356,384, the entire teachings of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Brightness enhancing films (BEF) have been used in lighting panels fordirecting light from lighting fixtures through luminaires and laptopcomputers displays. The brightness enhancing films, which can havelinear prisms, diffuse light with a desired directionality. Often thefilms have been used in combination with a fluorescent light source. Thefilms have had partial success in improving luminair or displaybrightness by controlling the angle at which light emerges. However, aneed still exists for improved control of lighting and enhancement ofbrightness for laptop computer screens.

SUMMARY OF THE INVENTION

The present invention includes a light collimating film having asheeting having a first side and a second side, wherein the first sideincludes a series of linear optical elements having a primary axisrunning the length of the optical elements, and the second side includesa plurality of subwavelength structures being oriented at about 90degrees relative to the primary axis of the linear optical elements. Thesubwavelength structures can include linear moth-eye structures. In oneembodiment, the linear optical elements are linear prisms having anincluded angle in the range of between about 60 and 120 degrees. Inanother embodiment, the linear optical elements include lenticularlinear elements. In a particular embodiment, the prisms have an includedangle of about 88 degrees. In another particular embodiment, the prismshave an included angle of about 89 degrees.

In another embodiment, the invention includes a back lighting displaydevice having a lighting device, a display panel, and a sheeting havinga first side and a second side, wherein the first side includes a seriesof linear prisms having peaks, and the second side includes a pluralityof subwavelength structures, the subwavelength structures being orientedat about 90 degrees relative to the peaks of the linear prisms.

In a further embodiment, the invention includes a light collimatingstructure having a first collimating film having a first surface with aplurality of linear moth-eye structures thereon and a second surfacewith first linear prisms having peaks, the linear moth-eye structuresbeing oriented at about 90 degrees relative to the peaks of the firstlinear prisms, and a second collimating film having a first surface witha plurality of linear moth-eye structures thereon and a second surfacewith second linear prisms having peaks, the moth-eye structures beingoriented at about 90 degrees relative to the peaks of the second linearprisms.

A method of forming a light collimating film is also provided whichincludes the steps of forming a series of linear prisms, which includepeaks, on a first side of a sheeting, and forming a plurality of linearmoth-eye structures on a second side of the sheeting with the linearmoth-eye structures being oriented at about 90 degrees relative to thepeaks of the linear prisms. The method can further include the steps offorming a series of linear prisms, which also include peaks, on a firstside of a second sheeting, and forming a plurality of linear moth-eyestructures on a second side of the second sheeting with the linearmoth-eye structures being oriented at about 90 degrees relative to thepeaks of the linear prisms. In one embodiment, the first and secondsheetings are arranged such that the moth-eye structures of the firstsheeting face the moth-eye structures of the second sheeting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a backlighting system.

FIG. 2 illustrates a perspective view of a linear prism structure.

FIG. 3 illustrates a side view of the linear prism structure shown inFIG. 2.

FIG. 4 illustrates a cross-sectional view of a second embodiment of aback lighting system.

FIG. 5 shows a plot of reflectance as a function of angles of incidenceand polarization for a moth-eye structure with 3,300 grooves permillimeter at a light wavelength of 514.5 nm.

FIG. 6 shows a plot of reflectance as a function of angles of incidenceand polarization for a moth-eye structure with 3,300 grooves permillimeter at a light wavelength of 647.1 nm.

FIG 7 shows a plot of reflectance for a dielectric having an index ofrefraction and a smooth non-moth-eye surface.

FIG. 8 shows a theoretical plot of output from a uniform lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 90 degrees.

FIG. 9 shows a theoretical plot of output from a uniform lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 90 degrees.

FIG. 10 shows a theoretical plot of output from a uniform lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 75 and 95 degrees,respectively.

FIG. 11 shows a theoretical plot of output from a uniform lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 75 and 95 degrees,respectively.

FIG. 12 shows a theoretical plot of output from a uniform lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)linear prisms having a prism angle of 75 degrees.

FIG. 13 shows a theoretical plot of output from a uniform lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)linear prisms having a prism angle of 75 degrees.

FIG. 14 shows a theoretical plot of output from a cosine lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 90 degrees.

FIG. 15 shows a theoretical plot of output from a cosine lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 90 degrees.

FIG. 16 shows a theoretical plot of output from a cosine lightdistribution X-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 75 and 95 degrees,respectively.

FIG. 17 shows a theoretical plot of output from a cosine lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)pitch linear prisms having a prism angle of 75 and 95 degrees,respectively.

FIG. 18 shows a theoretical plot of output from a cosine light,distribution X-profile for one and two films of 0.0019 inch (48 μm)linear prisms having a prism angle of 75 degrees.

FIG. 19 shows a theoretical plot of output from a cosine lightdistribution Y-profile for one and two films of 0.0019 inch (48 μm)linear prisms having a prism angle of 75 degrees.

FIG. 20 illustrates a side view of a subwavelength opticalmicrostructure.

FIG. 21 shows a plot of relative response versus wavelength of light fora 0.002 inch (51 μm) thick film of polyester, 0.002 inch (51 μm) thickfilm of polyester with one side having moth-eye structures, 0.002 inch(51 μm) thick film of polyester with two sides having moth-eyestructures, and a reference with a detector located normal to thesurface of the film.

FIG. 22 shows a plot of relative response versus wavelength of light fora 0.002 inch (51 μm) thick film of polyester, 0.002 inch (51 μm) thickfilm of polyester with one side having moth-eye structures, 0.002 inch(51 μm) thick films of polyester with two sides having moth-eyestructures, and a reference with a detector located at an angle 30degrees from the normal to the surface of the film.

FIG. 23 shows a plot of light transmission versus angle from the normalof a 0.002 inch (51 μm) polyester film with and without a moth-eyestructure on one side at the zero and 90 degree profile.

FIG. 24 shows a plot of color versus angle from the normal of a 0.002inch (51 μm) thick polyester film with and without moth-eye structureson both sides observed at zero degree orientation.

FIG. 25 shows a plot of color versus angle from the normal of a 0.004inch (102 μm) thick polyester film with and without moth-eye structureson both sides observed at zero degree X-orientation and 90 degreeY-orientation.

FIG. 26 is a plot of luminance cross section versus observation anglefrom the normal at zero degree orientation of a film with moth-eyestructures having a period of about 0.2 μm and a height of about 0.4 μmand linear prisms with 95 degree included angle and a pitch of 0.0019inches (48 μm).

FIG. 27 is a plot of luminance cross section versus observation anglefrom the normal at 90 degrees orientation of a film with moth-eyestructures having a period of about 0.2 μm and a height of about 0.4 μmand linear prisms with a 95 degree included angle and a pitch of 0.0019inches (48 μm).

FIG. 28 is a plot of luminance cross section versus observation anglefrom the normal at zero degree orientation of a film without moth-eyestructures and linear prisms with 95 degree included angle and a pitchof 0.0019 inches (48 μm).

FIG. 29 is a plot of luminance cross section versus observation anglefrom the normal at zero degree orientation of a film without moth-eyestructures and linear prisms with 95 degree included angle and a pitchof 0.0019 inches (48 μm).

FIG. 30 shows a plot of light transmission versus angle from the normalof a film with 90 degree linear prisms having a pitch of 0.002 inches(51 μm) with and without moth-eye structures on the window side of thefilms.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. All percentages and parts are by weightunless otherwise indicated.

DETAILED DESCRIPTION OF THE INVENTION

With respect to the optical performance of a collimating film, it hasbeen found that for individual AMLCD (active matrix liquid crystaldisplay) back lighting system designs, the optical efficiency of theparticular lamp, waveguide and diffuser system can be improved bydesigning a collimating film to maximize the use of the diffraction andrefraction effects. For example, as shown in FIG. 1, a back lightingsystem 10 includes a light source 12 and light reflector 14. Lightsource 12 can be a fluorescent light, incandescent light or othersuitable light source. Waveguide 16, which is for directing light out ofback lighting system, can be formed of a transparent solid material andis often wedge shaped. On one side of waveguide 16 is waveguidereflector 18 formed of a specular material, such as aluminum or a coatedwhite surface, for reflecting light back to waveguide 16. Waveguidereflector 18 can be curved or flat. Diffuser 20 is a film that diffusesthe light from the waveguide into a substantially uniform distribution.An example of a suitable diffuser is a randomly textured surface orgradient index film or engineered diffractive structure.

Above diffuser 20, first collimating film 22 has moth-eye structure 24on a first side adjacent waveguide 16. Second side of first collimatingfilm 22 has prism structure 25. An optional abrasion reduction layer 26is between first collimating film 22 and second collimating film 28. Theabrasion reduction layer can have a moth-eye structure on one or twosurfaces to improve performance. Second collimating film 28 has moth-eyestructure 30 on a first side adjacent first collimating film 22 andprism structure 32. Prism structure 32 of second collimating film 28 canbe oriented in the same direction as the prisms on first collimatingfilm 22. Alternatively, it may be offset by rotating the prismorientation up to about 180 degrees. In a preferred embodiment, thesecond collimating film is rotated about 90 degrees with respect to thefirst collimating film to reduce moire fringe formation and improve theuniformity of the exiting light distribution. Above the secondcollimating film is a liquid crystal display 34. A collimating filmwhich has linear prisms designed with a tilt, size and included anglewhich match the light source, waveguide and diffuser properties providesenhanced performance. The advantages of employing linear prism arrayswith included angles which range from 95 degrees to 120 degrees providesa light distribution which is optimized for viewing angles of a computerscreen. The included angle is considered the top angle of a triangularlinear prism structure.

An example of a linear prism film is shown in a perspective view in FIG.2 and in a side view in FIG. 3. Linear prism film 40 has prism surface42 and window surface 44 and is formed of a transparent polymericmaterial. Prisms 46 have sides 48 with peaks 50 and valleys 52. Thepitch (p) of the prisms 46 is measures from valley 52 to next valley 52.The pitch can be in the range of between 0.001 and 0.003 inches (25 and76 μm). The height (h) of the linear prisms is measured by the verticaldistance from the valley 52 to peak 50. The height can be in the rangeof between 0.0003 and 0.0015 inches (7.6 and 38 μm). The included angle(∝) is measured between the two sides that meet at peak 50. The angle(∝) can range from about 60 to 120 degrees. In a preferred embodiment,the angle (∝) is in a range of between about 60 and 85 degrees orbetween about 95 and 120 degrees. Sides 48 on each side of the peak 50can be the side length (l) from valley 52 to peak 50 to form anisosceles triangle. Alternatively, the sides can have different lengths,thereby tilting or canting the prisms. The tilting angle (β) of theprisms is between the optical axis 54 and a line 56 perpendicular to thewindow side 44. The prisms can be tilted in the range of between about−44 and +44 degrees. In a preferred embodiment, the tilting is aboutseven degrees.

In other embodiments, prism structures 22, 32 can include lenticularlinear elements, such as disclosed in U.S. Pat. No. 5,592,332, issued toNishio et al. on Jan. 7, 1997.

Another embodiment of the present invention is shown in FIG. 4. A backlighting system 100 includes a light source 102 and a light reflector104. Waveguide 106 can be formed of a transparent solid material and ispreferably wedge shaped. Adjacent to the first side 108 of waveguide 106is waveguide reflector 110 formed of a specular material. The reflector110 is spaced slightly away from surface 108 to allow total internalreflection at surface 108 to take place. First side 108 can be steppedin shape. Second side 112 of waveguide 106 is on the opposite side awayfrom waveguide reflector 110. Second side 112 has moth-eye structures114.

Above waveguide 106, first collimating film 116 has first prismstructure 118 with peaks 120 pointed toward waveguide 106 and firstmoth-eye structures 122 on the window side of first prism structure 118.Preferably, the peaks of linear prisms on first collimating film 116 runparallel to light source 102. Above first collimating film 116, secondcollimating film 124 has second moth-eye structure 126 and second prismstructure 128. Peaks 130 of second prism structure 128 point away fromwaveguide 106. Preferably, the peaks 130 of second prism structure 128is oriented in a non-parallel direction to peaks 120 of first prismstructure 118. A more preferred orientation is 90 degrees.

In one embodiment, the first moth-eye structures 122 are oriented atabout 90 degrees relative to the peaks of the linear prisms on the firstcollimating film 116. In one embodiment, angle (∝) of the linear prismson the first collimating film 116 is about 88 degrees. In anotherembodiment, angle (∝) is about 89 degrees.

The second moth-eye structures 126 can be oriented at about 90 degreesrelative to the peaks of the second collimating film 124. In oneembodiment, angle (∝) of the linear prisms on the collimating film 116is about 88 degrees. In another embodiment, angle (∝) is about 89degrees. The moth-eye structures 122, 126 are oriented at about 90degrees relative to the linear prisms of respective collimating films116, 124 to minimize, and preferable eliminate, the deep blue to deepgreen color that is produced by light resonance, and which is visible,at wide entrance angles.

In one embodiment having the moth-eye structures 122, 126 oriented 90degrees relative to the linear prisms of respective collimating films116, 124, only a small amount of green color shows at very wide viewingangles, for example, 70 to 80 degrees from normal. In contrast, when themoth-eye structures 122, 126 are oriented parallel to the linear prismsof respective collimating films 116, 124, the off axis color becomes avery vivid purple-blue at a viewing angle of about 45 degrees fromnormal. In some applications, this vivid color may be desirable where anarrow useable viewing is needed, such as in security/privacyapplications.

In the embodiment having 88 degree linear prisms on collimating films116, 124 with moth-eye structures 122, 126 oriented at about 90 degreesrelative to respective linear prisms, a 4% performance increase overcommercially available brightness enhancing films having 90 degreeprisms has been realized.

In the embodiment having 89 degree linear prisms on collimating films116, 124 with moth-eye structures 122, 126 oriented at about 90 degreesrelative to respective linear prisms, a 6% performance increase overcommercially available brightness films having 90 degree prisms has beenrealized.

The performance of TIR (total internally reflecting) films, often calledBEF (brightness enhancing film), which are used to increase the lightoutput from back lighting systems in AMLCD flat panel displays can beimproved by changing the tilt angle of the linear prism, the linearprism included angle and also the pitch of the linear prism array. Afurther improvement can be made by making the film monolithic orpolylithic. A monolithic film removes one material interface (at thesubstrate) and improves optical transmission. In the case of thepolylithic film, a diffuser can be incorporated into the film structuresaving the need to fabricate a separate diffuser and dependent on thedegree of collimation required.

A fine pitch of a linear corner cube prism structure provides excellentperformance as a first layer in a back lighting system if a diffuser isnot used between the top smooth surface of a waveguide and a flatsurface of the linear micro corner cube sheet. A fine pitch, preferablyin the range of between about 0.00005 and 0.0001 inches (1.3 and 2.5μm), of the corner cube array helps to spread the refracted andretroreflected light by diffraction creating increase diffusion ofrecycled light. In a more preferred embodiment, the pitch is about0.000075 inches (1.9 μm). The refracted and retroreflected light isspread by one to two degrees depending on the accuracy of the linearcorner cube array dihedral angles. This spreading is then increased bydiffuse structures on the second surface of the waveguide creating asmooth diffuse light pattern without the need of the diffuser betweenthe waveguide and linear corner cube collimating sheet. In addition, thegroove pattern in the linear corner cube array is oriented in directionswhich do not modulate with the diffuse dot pattern on the rear of thewaveguide. Therefore, moire fringes are not created. A surface isdisclosed in U.S. Pat. No. 5,600,462, issued to Suzuki et al. on Feb. 4,1997, the teachings of which are incorporated herein by reference, whichemploys a rough structure (0.004 inches, 10 μm) for performing diffusetransmission to create a “ground glass-type diffusion.” Also, it hasbeen found that the addition of one or two 95 degree linear prismsheet(s) with 0.0019 inch (48 μm) pitch above the fine pitch linearcorner cube sheet and with the smooth surface oriented toward the cornercube array further enhances the brightness. The second linear prismsheet is oriented about 90 degrees with respect to the first sheet.

The materials that work well for optical microstructured films are anultraviolet cured polymers bonded to a polyester substrate, which canhave abrasion resistance which is important during handling of thecollimating films. If the prism tips are damaged during handling, theresulting display can have fine lines that appear as less bright thansurrounding areas on axis and brighter than surrounding areas off-axis.The films can be formed of suitable polymers such as polycarbonate. Thefilms can be constructed from a polycarbonate material, acrylic, orother suitable material, such as disclosed in U.S. Pat. No. 5,396,350,issued to Beeson et al. on Mar. 7, 1995, the teachings of which areincorporated herein by reference.

An abrasion reduction sheeting, such as a thin polypropylene film orsimilar material can be placed in between the collimating film layers tohelp to reduce any effect from abrasion without losing significantbrightness. Subwavelength visible light moth-eye structures can be usedon these overleaf films to effectively eliminate Fresnel reflectionlight losses. The softer films do not abrade the linear prism peaks aseasily as hard films. A semi-soft substrate, such as a polyvinylchloride film can be used in place of the polyester substrate to makecollimating films and reduce abrasion. However, one must be careful ofout-gassing and resulting surface contamination which can occur withpolyvinyl chloride.

A linear non-isosceles prism array tilted or canted in the range ofbetween about −45 and +45 degrees and preferably at seven degrees,having a 95 degree included angle and a 0.0019 inch (48 μm) pitch as afirst layer and a linear isosceles prism (with zero tilt), having a 95degree included angle and a 0.0019 inch (48 μm) pitch as a second layercan significantly improve the amount of light that is directed throughan AMLCD to the angles (geometry's) desired for optimum user viewingangles. The tilt or canting of the optical axis of the first layerlinear prism array corrects the skewed direction of the lightdistribution coming from the waveguide and diffuser. A 0.0019 inch (48μm) pitch can cause diffraction spreading, which smooths the lightdistribution and maximizes the light directed toward the angles mostbeneficial or desired for a AMLCD display user. The 95 degree includedangle further optimizes the field of view of the light distribution forthe display user while still recycling light which is headed in theincorrect direction back into the display where it is used again.

Therefore, a preferred collimating film combination for a wedgewaveguide includes a first collimating film which has prisms tilted tocorrect for the skew created by the waveguide wedge and diffuser layersand has a prism angle designed to maximize the user field of view plus asecond collimating film oriented at 90 degrees to the first and with asymmetrical linear prism pattern. In the second collimating film theprisms can be tilted uniformly in both directions (tilt every otherprism in the opposite direction) to have a prism angle that optimizesthe user field of view for this axis.

Further, performance change can be achieved by combining the diffuserinto the first zone of the first collimating film to eliminate one filmcomponent. However, the focusing effect of the first surface is lost.The diffuser can be made by employing textured films and casting thelinear prisms onto the smooth side of the film, by rotary screenprinting a diffuse layer onto the polyester tie coat prior to castingthe linear prisms onto the diffuse layer (in this embodiment the diffuselayer is sandwiched between the linear prisms and the substrate film),by rotary screen printing a diffuse layer onto a carrier film and thencasting linear prisms onto the diffuse layer. The prism and diffuselayer can be made of the same material and finish cured together, byadding particles into the tie coat prior to casting the linear prismsonto the tie coat and by dispersing particles in the substrate sheetfollowed by casting linear prisms onto the substrate sheet.

The addition of the moth-eye structure to the window side of thecollimating films increases the system brightness by about 6% to 8%,which is significantly brighter (by 10% to 12%) than the previouslyknown brightness enhancing film systems with a similar pitch.

These improved results are believed to be due to a combination ofmicrostructured optical effects. The uniform white light, such as afluorescent bulb which causes this light to have a cool appearancebecause of the blue shift, has distribution coming from the diffuserwhich is incident on the first layer moth-eye surface. At angles ofincidence of +/−60 degrees, 2% or less of the light is reflected at thefirst layer moth-eye to air interface. Plots of the reflectance areshown in FIGS. 5 and 6 for a subwavelength microstructure having 3,300grooves per millimeter of light having wavelengths of 514.5 nm and 647.1nm, respectively. The S line represents light perpendicular to the planeof incidence, and the P line represents light parallel to the plane ofincidence. Shown in FIG. 5, the average reflectance (linear averagebetween S and P lines) is about 0.8% at 60 degrees and shown in FIG. 6,the average reflectance is about 2% at 60 degrees. This is compared toan average of about 10% of the light that is reflected at a smoothnon-moth-eye surface at a 60 degree angle of incidence. FIG. 7 shows aplot of an average of about 10% reflectance at a 60 degree incidentangle. Also at normal incidence, a typical 4% reflectance due to asmooth surface is reduced to less than 1% with a moth-eye structure.

At angles of incidence that are greater than 75 degrees, a green andthen a blue color can be observed. The color is a result of diffractionscattering as the short wavelengths enter the moth-eye structure from anangle which causes the aperture of the moth-eye elements to becomediffractive and resonate. This diffraction scattered light is processedby the linear prism film differently than the light that passes througha non-moth-eye smooth surface. In this embodiment, the green to bluelight is more uniformly distributed throughout the film creating a moreuniform illumination. A significant amount of green light is light pipedby total internal reflection within the film and is partially filteredout of the light that becomes available to illuminate an LCD panel.Different size (frequency and amplitude) moth-eye structures can be usedto create different illumination effects depending on the light sourceand optical components used in the illumination system. By changing thesize of the moth-eye structures in the range from sub-wavelength tolarger than wavelength scale structures, for example, moth-eyestructures having a period of between about 0.15 and 10.0 micrometers,the diffraction properties of the surface can be optimized to helpsmooth the resulting light distribution and improve wide angle lightdistribution.

After the light has passed through the first moth-eye layer, it iscollimated to about 42 degrees and the 95 degree linear prism secondsurface of the first film layer through refraction collimates the lightto approximately +/−30 degrees. Then the light enters the moth-eyesurface on the first surface of the second layer film where it isfurther collimated by refraction. The majority of the light is at +/−30degrees from the normal as it enters the moth-eye surface and passesthrough the moth-eye layer with little intensity loss. The light passesthrough the second layer film and is further redirected throughrefraction and recycling by the 95 degree linear prism structure. The 95degree prism shape helps to recycle any of the light that is stilltraveling at wide angles of incidence. This light eventually emergesfrom the lighting system within a final +/−29 degree light distributionin both the X and Y axes.

If two crossed 75 degree linear prism films with moth-eye smoothsurfaces are used in the illumination system, a light intensitydistribution width of +/−18 degrees and an intensity of 2.63 can beachieved. The light intensity distribution X-profile and Y-profile foruniform light through a film with 90 degree, 75 degree and 95 degreeprism films are shown. FIGS. 8 and 9 show plots of output uniform lightdistribution of the X-profile and Y-profile, respectively, for one andtwo films of 0.0019 inch (48 μm) pitch for linear prisms having a prismangle of 90 degrees. FIGS. 10 and 11 show plots of output for uniformlight distribution X-profile and Y-profile, respectively, for one andtwo films of 0.0019 inch (48 μm) pitch for linear prisms having a prismangle of 75 and 95 degrees, respectively. FIGS. 12 and 13 show plots ofoutput for uniform light distribution X-profile and Y-profile,respectively, for one and two films of 0.0019 inch (48 μm) linear prismshaving a prism angle of 75 degrees. FIGS. 14 and 15 show plots of outputcosine light distribution of the X-profile and Y-profile, respectively,for one and two films of 0.0019 inch (48 μm) pitch for linear prismshaving a prism angle of 90 degrees. FIGS. 16 and 17 show plots of outputfor cosine light distribution X-profile and Y-profile, respectively, forone and two films of 0.0019 inch (48 μm) pitch for linear prisms havinga prism angle of 75 and 95 degrees respectively. FIGS. 18 and 19 showplots of output for cosine light distribution X-profile and Y-profile,respectively, for one and two films of 0.0019 inch (48 μm) linear prismshaving a prism angle of 75 degrees. Additional optimization of theangles allows a near +/−10 degree intensity distribution. Onedisadvantage with this configuration is an approximate +/−2.0 degreevoid that appears at the center of the light intensity distribution.This effect is visible in FIGS. 12 and 13. Slight curvature orpositive-negative canting in the prism facets can reduce this void.

The application of a moth-eye structure to the smooth surface of thelinear prism films improves significantly the light collimatingcapability of the films by increasing light throughput at the moth-eyestructured surface and redirecting wide incident angle light rays.Diffraction effects also play a significant role in the improvedperformance of the system. The resulting color of the backlight assemblyis warmer in appearance than the same assembly without the addition ofthe moth-eye structures. This color shift can have a beneficial effecton the contrast within the final back light display.

As shown in FIG. 20, the moth-eye structure applied preferably has anamplitude (A) of about 0.4 micrometers and a period (P) of less thanabout 0.2 micrometers. The structure is sinusoidal in appearance and canprovide a deep green to deep blue color when viewed at grazing angles ofincidence. Preferably, the amplitude is about three times the period toprovide a three to one aspect ratio.

FIGS. 21 and 22 show a plot of the improvement in transmission bywavelength for 0.002 inch (51 μm) thick PET having moth-eye structureson one side and having moth-eye structures on both sides at zero degreesand at 30 degree angles from the normal, respectively. The moth-eyestructures have a period of about 0.2 μm and a height of about 0.4 μm.The reference is a uniform light distribution coming from a diffuserpositioned above the waveguide. FIG. 23 shows a plot of the improvementin transmission by angle from the normal for 0.002 inch (51 μm) PET withmoth-eye structures. In this figure, the fluorescent tube light bulb isat a +80 degree position for the 90 degree orientation. FIG. 24 shows aplot of the color shift which occurs for 0.002 inch (51 μm) thick PETwith moth-eye structures on one side and with moth-eye structures onboth sides. FIG. 25 shows a plot of the color shift that occurs for0.004 inch (102 μm) thick PET with 95 degree linear prisms on the sideaway from the diffuser and with and without moth eye on the side closeto the diffuser. For all measurements, the samples were placed on top ofthe diffuser in a standard LCD back light assembly and the PhotonResearch detector, Model PR650 was supported eighteen inches (45.7 μm)above the part surface.

The moth-eye structure provides anti-reflection properties to thepreviously smooth light entrance surface of the substrate even atentrance angles that are near grazing incidence. The moth-eye structureis more effective than standard thin film anti-reflection coatings atwide angles of incidence especially angles of incidence beyond 30degrees up to 80 degrees. This characteristic causes many types ofoptical microstructure films including linear prism films to processlight very differently than the standard linear prism collimating filmswhich have smooth entrance surfaces with or without standardanti-reflection thin film (vacuum deposited or liquid applied) coatings.The addition of the moth-eye structures helps to more efficientlyrecycle light and also redirects the normally reflected grazing angleincidence rays into the optical microstructure (such as linear prisms)sheet where the rays are refracted, reflected or retroreflecteddepending on the respective angles of incidence. This moth-eyeimprovement concept can be added to many types of brightness enhancementfilms (BEF). An advantage is that functional optical microstructures canbe applied to both sides of a film or substrate.

A moth-eye anti-reflection surface is one in which the reflection oflight is reduced by the presence of a regular array of smallprotuberances covering the surface. The spacing of the protuberances isless than the wavelength of light for which anti-reflection is sought. Amoth-eye surface can be understood in terms of a surface layer in whichthe refractive index varies gradually from unity to that of the bulkmaterial. Without such a layer the Fresnel reflection coefficient at aninterface of two media is equal to ((n₁−n₂)/(n₁+n₂))², where n₁ and n₂are the refractive indices of the media. However, if there is a gradualchange of index, net reflectance can be regarded as the result of aninfinite series of reflections at each incremental change in index.Since each reflection comes from a different depth from the surface,each has a different phase. If a transition takes place over an opticaldistance of λ/2, all phases are present, there is destructiveinterference and the reflectance falls to zero.

When the height of the protuberance (h) is significantly less than thewavelength (λ), the interface appears relatively sharp and thereflectance is essentially that of a discontinuous boundary. As theratio of h/λ increases, the reflectance decreases to a minimum value atabout h/λ=0.4. Further increases in h/λ show a series of successivemaxima and minima, but the value does not again approach that of a sharpinterface. The details of the curve shown in FIG. 20 vary depending onthe profile of the change of the index of refraction, but if thethickness is of the order of half a wavelength or more the reflectanceis considerably reduced. The spacing of the protuberances should besufficiently fine to avoid losses by diffraction. Preferably, it shouldbe less than the shortest wavelength involved divided by the refractiveindex of the material.

It is important that the spacing d between the peaks of theprotuberances on the moth-eye surface is sufficiently small that thearray cannot be resolved by incident light. If this is not the case, thearray can act as a diffraction grating and, although there may well be areduction in the specular reflection (zero order), the light is simplyredistributed into the diffracted orders. In other words, we requirethat d<λ for normal incidence and d<λ/2 for oblique incidence if forreflection only, and that d<λ/2n in the case of transmission wherediffraction inside the material is suppressed.

For a given moth-eye surface, where the height of the protuberances is hand the spacing is d, the reflectance is expected to be very low forwavelengths less than about 2.5 h and greater than d at normalincidence, and for wavelengths greater than 2 d for oblique incidence.Preferably, the spacing is as close as possible, and the depth as greatas possible, in order to give the widest possible bandwidth. Forexample, a h/d ratio is preferably about three.

The moth-eye effect should not be confused with that of reducing thespecular reflectance by roughening. Roughness merely redistributes thereflected light as diffuse scattering and degrades the transmittedwavefront. With the moth-eye structure, there is no increase in diffusescattering, the transmitted wavefront is not degraded and the reductionin reflection gives rise to a corresponding increase in transmission.

The moth-eye structure has many advantages. There is no extra coatingprocess necessary. The structure can be transferred to the sheet by apressure molding process, such as with a Fresnel structure. Thereflection reduction does not depend on the wavelength. There is only alower limit (on the ultraviolet side of the spectrum) set by thestructure period. If the wavelength is too small compared to the period,the light is diffracted. In regard to angular dependence, withconventional anti-reflective coatings, the transmission curve shiftswith the light incidence angle. With the moth-eye structure, thecritical wavelength for diffraction shifts to higher values, but thereare no changes above this wavelength. Another advantage for moth-eyestructures is that there are no adhesion problems between lens andgradient layer because it can be one bulk material. From a high incidentangle, the surfaces can appear blue or violet.

To form a moth-eye structure, the structure is first produced on aphotoresist-covered glass substrate by a holographic exposure using anultraviolet laser. A suitable device is available from HolographicLithography Systems of Bedford, Mass. 01730. An example of a method isdisclosed in U.S. Pat. No. 4,013,465, issued to Clapham et al. on Mar.22, 1977, the teachings of which are incorporated herein by reference.This method is sensitive to any changes in the environment, such astemperature and dust, and care must taken. The structure is thentransferred to a nickel shim by an electroforming process. In apreferred embodiment, the shims are about 300 micrometers thick or less.

The moth-eye structures can be made one dimensional in a grating typepattern. In this embodiment, the structure has a nearly rectangularprofile, which means they have no gradient layers, but more of a onelayer anti-reflective coating with a lowered refractive index in thestructure region. Control of the grating depth is important as iscontrol of thickness for the evaporated layers. Control of depth andthickness is achieved by maintaining uniformity of beam exposure,substrate flatness and exposure time.

A two-dimensional structure is formed by two exposures with a linearsinus-grid, turned by 90 degrees for the second exposure. A third typeof structure is formed by three exposures with turns of 60 degrees toprovide a hexagonal or honeycomb shape.

When measured at a four inch (10.2 cm) distance from the display PhotonResearch Model No. PR650, the results with two 95 degree linear prismfilms each having a moth-eye structure on the previously smooth sideshow about the same brightness on axis as two 90 degree BEF films, alarge improvement in brightness off axis in both vertical and horizontalaxis and a warmer color to the light emerging from the display. In FIGS.26, 27, 28 and 29, the total integrated light intensity for the 95degree prisms with moth-eye structure films is 6,686.8 lm/m² with amaximum of 4,460 cd/m² and a minimum of 554.0 cd/m². For the 90 degreeprisms without moth-eye structure films, the integrated light intensityis 5,698.8 lm/m² with a maximum of 4,685.0 cd/m² and a minimum of 295.9cd/m².

Through analysis and experimental results, a preferred embodimentincludes a 75 degree linear prism film can be used as the first layerabove a uniform light output diffuser to collimate the light to about a+/−30 degree angle. The prism grooves in this first layer are orientedparallel to the light source that illuminates the waveguide which isbelow the diffuser. On top of this film can be a 95 degree linear prismfilm which is oriented at 90 degrees with respect to the 75 degree filmto collimate the light to about +/−25 degrees with a small percentage ofthe light at +/−30 degrees as shown in FIGS. 10, 11, 16 and 17. Thefinal intensity of the collimated light is excellent and comparable toresults obtained with two crossed 90 degrees BEF films, as shown inFIGS. 8, 9, 14 and 15. These 90 degree BEF films do not allow for therecycled component (Note that there is no recycled light with the 75degree film) but allowing for the recycled light the peak intensitiesbecome 2.15 for the 75 degree plus 95 degree collimating films and 2.06for the BEF films. The prism apex angle of the second 95 degree film canbe increased to about 100 degrees if the spread in the collimated lightbeam is too narrow.

FIG. 30 shows a comparative plot of light transmission versus angle fromthe normal of a film with 90 degree linear prisms having a pitch of0.002 inch (51 μm) with moth-eye structures on the window side of thefilm and a film with 90 degree linear prisms having a pitch of 0.002inch (51 μm) without moth-eye structures on the side of the film. Thecomparative plot shows a substantial improvement in transmission,particularly at zero degrees, when employing a moth-eye structure on thewindow side of the film as compared to a similar film without a moth-eyestructure.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A light collimating film comprising a sheetinghaving a first side and a second side, wherein said first side includesa series of linear optical elements having a primary axis running thelength of the optical elements, and said second side includes aplurality of moth-eye structures, said moth-eye structures beingoriented at about 90 degrees relative to the primary axis of said linearoptical elements.
 2. The light collimating film of claim 1 wherein saidlinear optical elements include linear prisms that run the width of thesheeting.
 3. The light collimating film of claim 1 wherein said linearoptical elements include linear prisms having triangular prisms arrangedside-by-side.
 4. The light collimating film of claim 3 wherein saidtriangular prisms include a top angle in a range of between about 60 and120 degrees.
 5. The light collimating film of claim 4 wherein saidlinear prisms include triangular prisms that are isosceles in shape. 6.The light collimating film of claim 3 wherein said triangular prismsinclude a top angle in a range of between about 60 and 85 degrees. 7.The light collimating film of claim 3 wherein said triangular prismsinclude a top angle in a range of between about 95 and 120 degrees. 8.The light collimating film of claim 3 wherein said triangular prismsinclude a top angle of about 88 degrees.
 9. The light collimating filmof claim 3 wherein said triangular prisms include a top angle of about89 degrees.
 10. The light collimating film of claim 1 wherein saidlinear optical elements are pitched at regular intervals.
 11. The lightcollimating film of claim 1 wherein said linear optical elements includelenticular linear elements.
 12. The light collimating film of claim 1wherein said linear optical elements are pitched in the range of betweenabout 0.0127 and 6.35 millimeters.
 13. The light collimating film ofclaim 1 wherein said moth-eye structures include linear moth-eyestructures.
 14. The light collimating film of claim 13 wherein saidsecond side includes linear moth-eye structures spaced about 0.15micrometer apart.
 15. The light collimating film of claim 13 whereinsaid linear moth-eye structures have depth to width ratio in range ofbetween about one and to three.
 16. The light collimating film of claim13 wherein said linear moth-eye structures include a period in the rangeof between about 0.15 and 10.0 micrometers.
 17. A back lighting displaydevice, comprising: a) a lighting device; b) a display panel; and c) asheeting having a first side and a second side, wherein said first sideincludes a series of linear prisms having peaks, and said second sideincludes a plurality of linear moth-eye structures, said linear moth-eyestructures being oriented at about 90 degrees relative to the peaks ofsaid linear prisms.
 18. The display device of claim 17 furthercomprising a second sheeting having a first side and a second side,wherein said first side includes a series of linear prisms having peaks,and said second side includes a plurality of linear moth-eye structures,said linear moth-eye structures being oriented at about 90 degreesrelative to the peaks of said linear prisms.
 19. The display device ofclaim 17 wherein said linear prisms include triangular prisms having atop angle of about 88 degrees.
 20. The display device of claim 17wherein said linear prisms include triangular prisms having a top angleof about 89 degrees.
 21. A light collimating structure, comprising: a) afirst collimating film having a first surface with a plurality of linearmoth-eye structures thereon and a second surface with first linearprisms having peaks, the linear moth-eye structures being oriented atabout 90 degrees relative to the peaks of said first linear prisms; andb) a second collimating film having a first surface with a plurality oflinear moth-eye structures thereon and a second surface with secondlinear prisms having peaks, the linear moth-eye structures beingoriented at about 90 degrees relative to the peaks of said second linearprisms.
 22. The light collimating structure of claim 21 wherein thelinear moth-eye structures of the first collimating film and the linearmoth-eye structures of the second collimating film face each other. 23.A method of forming a light collimating film, comprising: forming aseries of linear prisms on a first side of a sheeting, the linear prismsincluding peaks; and forming a plurality of linear moth-eye structureson a second side of the sheeting with the linear moth-eye structuresbeing oriented at about 90 degrees relative to the peaks of the linearprisms.
 24. The method of claim 23 further comprising the steps of:forming a series of linear prisms on a first side of a second sheeting,the linear prisms also including peaks; and forming a plurality oflinear moth-eye structures on a second side of the second sheeting withthe linear moth-eye structures being oriented at about 90 degreesrelative to the peaks of the linear prisms.
 25. The method of claim 24further comprising the step of arranging the first and second sheetingssuch that the moth-eye structures of the first sheeting face the linearmoth-eye structures of the second sheeting.